This invention relates to pacemaker sense circuits and, more particularly, to a single-amplifier circuit which functions as both a delta modulator and a bandpass filter.
A pacemaker, whether implantable or external, typically includes a bandpass filter as part of the sense circuit. The center frequency is usually in the 30-80 Hz range, and the filter usually has a value of Q which is greater than 1. Bandpass filter designs for use in pacemaker sense circuits are well-known to those skilled in the art. It is usually required to use two capacitors and two resistors in the filter. In order to achieve a Q of 2, the preferred value (too high a value gives rise to ringing, and too low a value sacrifices gain and gives rise to poorer noise rejection), it is necessary to include an amplifier as part of the filter.
Another technique involved in sensing that has been the subject of several patents in recent years is that of delta modulation. Reference is made, for example, to U.S. Pat. Nos. 4,448,196; 4,466,440; 4,509,529 and 4,527,133.
A delta modulator, in its most basic form, requires not only a capacitor which can be charged and discharged, but also an amplifier for operating on the signal and the potential across the capacitor in order to determine whether the capacitor should be charged or discharged during the current sampling period. (There are other components, such as two current sources, which a delta modulator requires, but these other components have no bearing on the subject invention.) Taken together, a delta modulator and a bandpass filter would appear to require a total of three capacitors, two resistors and two amplifiers.